There are several steps involved in the production of proteins in cells and any of these are subject to selective regulation in cell differentiation. The proposed work will investigate the possibility that the differential expression of a family of plasma membrane polypeptides in different cell types of the cellular slime mold Dictyostelium discoideum is regulated by a post-translational mechanism. The hypothesis predicts that a precursor polypeptide is constitutively expressed in all cell types but is only glycosylated and/or phosphorylated in selected cell types. We have serological and cell fractionation data in support of this hypothesis for one member of the family. Our new experiments intend to document the existence of a single primary polypeptide prior to processing, identify the chemistry of the processing change, establish a precursor-product relationship, establish the relative contributions of synthesis and turnover, verify the intracellular localizations, and generalize the findings to other plasma membrane (apparently cell surface) polypeptides. Circumstantial evidence suggests that members of this family of glycopolypeptides are involved in the regulation of cell differentiation in the D. discoideum slug. The results of our experiments will be interpreted in the context of a cell differentiation mutant whose primary lesion may involve the post-translational processing of one or more members of this glycopolypeptide family.